Method of forming a screen on material



Dec. 1, 1936. J. c. BATCHELOR 1 3 METHOD OF FORMI'ING i; SCREEN ON MATERIAL I Filed Aug. 27, 1930 V- Figg- 1f 1 W6 w m m M M 5 d R O. J h.

Patented Dec. 1, 1936 UNITED STATES PATENT OFFICE John 0. Batchelor, Collingswood, N. 1.,

assignor,

by mesne assiznmcnts, to Radio Corporation of America, Delaware New York, N.]Y., a corporation of Application August 27, 1930, Serial No. 478,048 14 Claims. (01. 250-275) My invention relates to improvements in methods of forming a screen on material, and'more particularly a fluorescent and electrically con ductive screen on the end wall structure of a cathode ray tube.

In the art of construction of a cathode ray tube for television reception of the general type disclosed in co-pending application bearing Serial No. 407,652 and flied November 16, 1929, the problem has arisen of applying or forming a flucrescent screen or coating on the large end of the tube for scanning by the cathode ray developed by the electron gun at the small end. In this connection, it has been found to be most important that the fluorescent screen or coating be smooth, of an even depth or thickness throughout, and tenacious; that is, well applied and capable of remaining fixed to the wall structure of the tube at every point as originally ap-' plied thereto, so that no part or piece of the screen material will break away under normal operating conditions and careful handling. In actual tests of such a tube, undesirable spots on and differences in definition over different regicns of the received image have been partly attributed, in some cases, to extremely small dif ferences in depth or thickness at the correspond- ,ing regions of the screen, or extremely slight differences in sensitivity of these regions of the screen to the influence of the cathode ray.

In construction of the so-called gas-filled tube, wherein argon or some other suitable gas fills the same, tenacity of the fluorescent screen has been obtained by mixing with the pulverized fluorescent material, such as willemite, binding material such as sodium silicate, amyl acetate, or a suitable cellulose.

With the advent of the high vacuum cathode ray tube, which has been found to have important advantages over the gas-filled tube referred to, the difliculty of providing a satisfactory fluorescent screen has increased for the reasons, first, that unless great care is taken in evacuating and sealing off the tube, air molecules retained in the binding material eventually escape and destroy the orig nal high vacuum, which action causes ionization in such degree as to seriously interfere with the normal operating action of the tube, and, second, some means was found to be necessary to provide a, conductive path for the electrons, previously provided in the gasfil'ed tubes by ionization of the argon or other gas, from the fluorescent screen back to the oathode forming part of the electron gun, to prevent accumulation of electrons at any spot on the screen to which the cathode ray might be directed at any instant.

With all the foregoing in mind,it is one of the objects of the present invention to provide an improved method of forming a fluorescent screen on material, and particularly a fluorescent screen on the wall structure of a high vacuum cathode ray tube, the screen being readily formed or applied and having all the required characteristics in the way of smoothness, uniformity of depth or 1 evenness, and tenacity.

Another object is to provide an improved method of forming a screen of the character referred to .which is sufliciently electrically conductive to form part of a. conductive path for the electrons 15 back to the cathode, thereby preventing accumulation of electrons at any spot on the screen to which the cathode ray might be directed at any instant.

Other objects and advantages will hereinafter appear. a

In accordance with my invention, a fluorescent screen is formed on or applied to a surface of material, such as the end wall structure of a cathode ray tube, by covering the material or structure with a liquor such as water holding in suspension relatively small particles of fluorescent material, allowing the particles to settle out of suspension inthe liquor into adherence with the material or structure to form a fluorescent coating thereon, and removing the clear liquor by evaporation, pumping, by gradually tilting the material or structure to drain of! the clear liquor without disturbing or distorting the coating, or in any other suitable manner.

Further in accordance with my invention, an electrically conductive, fluorescent and translucent screen is formed on or applied to a surface of material, such as glass, by depositing on the surface a very thin coating of electrically con- 0 ductive material such as silver, the coating being thin enough to be substantially transparent, covering the coated surface with a liquor such as water holding in suspension relatively small par ticles of fluorescent material, allowing the par- 45- ticles to'settle to form a fluorescent coating over the silver .or other electrically conductive coating, and removing the clear liquor by draining or in any other suitable manner Still further in accordance with my invention,

an electrically conductive, fluorescent and translucent screen is formed on or applied to a surface of material, such as glass, by covering the surface with a suitable solution, such as can alkaline solution of silver nitrate containing an agent effective to reduce the silver nitrate to metallic silver, the solution holding in suspension relatively small particles of fluorescent material and being of such strength that a very thin and substantially transparent coating of silver or other suitable electrically conductive material is deposited on the surface, permitting the particles to settle out of suspension into adherence with the surface or the silver or other coating being simultaneously deposited thereon, and removing the solution in any suitable manner after the fluorescent particles have all settled.

Still further in accordance with my invention, an electrically conductive, fluorescent and translucent screen is formed on or applied to a surface of material, such as glass, by covering the surface with liquor such as water holding in suspension relatively small particles of fluorescent material and electrically conductive material such as silver, permitting the particles to settle and form an electrically conductive, fluorescent and translucent coating on the surface, and removing the clear liquor in any suitable manner such as by draining, therelation between the respective sizes of the particles of the two materials being so chosen that the rate of settling of the electrically conductive particles bears a predetermined relation to and is substantially greater than the rate of settling of the fluorescent particles, whereby that portion of the screen or coating adjacent the surface of the material contains the desired percentage of the electrically conductive particles, while the upper or surface portion of the screen or coating contains the desired percentage of the fluorescent particles.

Still further in accordance with my invention, an electrically conductive, fluorescent and translucent screen is formed on or applied to a surface of material, such as glass, by shaking pulverized fluorescent material in a suitable solution, such as an alkaline solution of silver nitrate containing an agent effective to reduce the silver nitrate to metallic silver, the solution being of suflicient strength to cause a relatively thin coating of silver or other electrically conductive material to be deposited over the surface of each particle, washing away the silver nitrate or other solution with water or any other suitable liquor to leave the silver-coated fluorescent particles suspended therein, covering the surface of the material with the liquor and permitting the silver-coated fluorescent particles to settle out of suspension into adherence with said surface in the form of a coating, and removing the clear liquor. My invention resides in the method and apparatus of the character hereinafter described and claimed.

For the purpose of illustrating my invention several embodiments thereof have been described in the accompanying specification, in.

which reference is made to the drawing, wherein Fig. 1 is a greatly enlarged, fragmentary, sectional view, illustrating a principle of operation;

Fig. 2 is an elevational, sectional view, illustrating one step in a method embodying the present invention;

Figs. 3, 4, and 5 are enlarged, fragmentary, sectional views, illustrating subsequent steps in the method;

Figs. 6, 7, and 8 are views similar to Figs. 3, 4, and 5, illustrating a modification;

Fig. 9 is a greatly enlarged, cross-sectional view of a particle of material, illustrating one step in a modified method embodying the present invention; and

Fig. 10 is an enlarged, fragmentary, sectional view of a cathode ray tube constructed in accordance with the present invention.

In Fig. 1, the end wall III of. a glass tube, such as might be used in construction of cathoderay apparatus of the general type disclosed in the co-pending application referred to above, is shown provided with a coating or screen H of fluorescent material on the surface exposed to the cathode ray which is deflected in the usual manner to the positions A, B, C, and D, for example. Ifhe fluorescent coating is shown as being rough and uneven, the hills and dales being exaggerated for the sake of clarity in the present explanation. It is a well known phenomenon that when a cathode ray strikes fluorescent material, the degree of fluorescence resulting, or the number of light photrons emitted by the material, is a function of both the intensity of the cathode ray at the instant and the depth or thickness of the spot on the fluorescent mate- .rial being struck by the ray at that instant.

With the foregoing in mind, and assuming that the cathode ray is of the same or substantially the same intensity in each of the scanning positions A, B, C, and D, it will be seen that the fluorescent spots, as observed from the exterior of the tube over the respective spots or regions a, b, c, and d, will all be of diiferent intensity or brightness. That is, when the ray is in position A it will strike a relatively thick spot on the irregular fluorescent coating 1 I, and a relatively bright spot will be observed over the region a. When the ray, remaining at the same intensity, is deflected to the position B, it will strike a relatively thin spot on the coating l l, and the light spot observed over the region b will be correspondingly less in intensity than the intensity of the preceding light spot over the region a. The observed image, made up of the light spots, such as the spots a, b, c, and (1, will therefore be somewhat indefinite, and at any rate, will not represent an exact light reproduction or conversion of the received picture impulses with which the intensity of the cathode ray corresponds. Results obtained in tests of different fluorescent screens indicate that even very small imperfections therein in the way of irregularities on the surface contribute in large part to imperfections in the light image developed for observation.

With reference now to Figs. 2 to 5, the reference numeral l2 designates a glass tube of the type generally used in construction of a cathode ray tube for television reception, the tube being provided on the interior surface of the side wall structure thereof with the silver coating I3 which extends for a short distance, as more clearly shown in Fig. 10, over the edge portion of the transparent end wall M.

In practicing my invention, a suitable reducing agent, such as an alcoholic solution of sugar,-is added to a three to five per cent alkaline solution of silver nitrate, whereby the silver solution is reduced to metallic silver in a period of time of about six to' seven minutes; Satisfactory results have been obtained by adding one part of the reducing agent to every twenty-five parts of the silver solution. Immediately after addition of the reducing agent, the silver solution is poured into the open end of the tube and the latter supported in the vertical position shown aoeasss stantially transparent coating ii of silver to be deposited on the end wall I. During this time a plug or stopper i6 is inserted into the open neck of the tube. After the thin silver coating I 6 has been deposited, the tube is turned on its side and rotated slowly and for such length of time as to permit the relatively heavy coating it of silver to be deposited. The stopper is then removed and the tube emptied of the liquid contents.

The tube i2 is then placed in the vertical position shown in Fig. 2 and a suitable clamp 9 applied to the neck thereof, the clamp being fixed on the free end of a shaft I 'I supported for rotation'about a horizontal axis to tilt the tube through the angle a: to the position indicated by the broken line showing.

The material for the fluorescent screen is prepared by pulverizing fluorescent material, such as willemite, by grinding ofl' relatively small particles from the material. The pulverized material is then screened through a mesh of suitable size, such as 270 per inch. The screened particles are thoroughly mixed to remove any lumps, and are then shaken into a suitable suspending liquor such as water. The very fine particles are floated ofi. The liquor designated by reference numeral it, with the sharp powder particles IQ of fluorescent material suspended therein, is then poured into the tube, the amount of liquor used being just suflicient to cover the surface of the end wall it with the liquor at a relatively small depth, such as one quarter of an inch, as indicated in Figs. 2, 3, and 4.

The tube I2 is allowed to remalnin the vertical position until all of the fluorescent particles have settled and adhered to the end wall I to form a fluorescent coating-or screen 20 over the thin silver coating is. The sharpness of the individual particles, caused by the grinding operation, insures that the same will adhere or stick to the end wall i4 and pack in interlocking relation with each other to make a strong screen.

When the fluorescent particles have settled completely to form the screen 26, the shaft it is rotated in a counterclockwise direction at a substantially constant rate of the order of six degrees in one minute. As the coated surface of the end wall It is thus tilted very gradually out of the horizontal plane, the clear liquor drains from the fluorescent coating 20, as indicated more clearly in Fig. 5, without disturbing or dislodging any of the small fluorescent particles making up the same.

When the tube has been rotated'through the angle :1:, all the clear liquor is will have been removed, leaving a smooth and tenaciousfluorescent coating 20 of uniform depth or thickness throughout.

Instead of draining off the clear liquor, the same may be removed by inserting a tube into the tube I2 and pumping or sucking the liquor out.

Another alternative is to permit the liquor to evaporate, for which purpose heat may be applied, or the tube placed under vacuum to hasten evaporation.

The amount of powdered fluorescent material placed in suspension in a given amount of water j or other liquor may vary widely,

depending upon particular requirements, but at any rateis sufllcient to insure for .a desired thickness.

The fluorescent particles should be free from grease and dirt. Grease and dirt may be removed from the fluorescent particles by washing fluorescent coating of the the same in ether before placing the particles in the water or other suspending liquor.

The fluorescent material is pulverized and screened so that the individual particles are of such size to cause the same to settle at a predetermined rate in the suspending liqusr. The rate of settling of the particles may be determined by the formula wherein v is the rate of fall or settling of a particle, r the radius of the particle, S the speciflc gravity of the particle, S the specific gravity of the suspending liquor, n the viscosity coeilicient of the liquor, and a the force of gravity.

The rate of settling may vary within wide limits, and should be made such as to permit the particles to settle evenly and pack themselves together into a solid, strong coating- After treatment by the method explained above, the end wall structure of the tube I2 is characterized by the fact that the same is translucent, fluorescent and electrically conductive. That is, the thin silver coating the translucent and fluorescent coating 20, provides an electrically conductive leakage path from any spot on the coating 20 to the heavy silver coating l3 which is maintained at a relatively high positive potential through a connection 2i leading to a suitable source of supply. Accumulation of electrons, at any spot on the screen 20 to which the cathode ray might be (11- The property of being electrically conductive may be given to the screen structure by mixing with the powdered fluorescent particles a powdered metal such as silver or any other suitable electrically conductive material, and placing this mixture in suspension in the water or other liquor and covering the surface of the tube l5 which backs end I4, as shown in Fig. 6, at which time the fluorescent particles 22 the suspending liquor l8, as indicated. It is proposed to proportion the two materials so that there will be in suspension about eighty per cent of the fluorescent material and about twenty per cent of the electrically conductive material.

This proportion may vary within wide limits, to meet difierent requirements. Since the specific gravity of the metallic particles is appreciably greater than that of the fluorescent particles, the rate of settling of the former will be correspondingly greater than that of the latter, and as the settling proceeds, the percentage of the metallic particles in the liquor adjacent the surface of the end wall I will exceed the percentage of fluorescent particles in this region, as indicated in Fig. 7. The particles of the two maand the metallic particles 23 will be uniformly distributed throughout terials will therefore finally settle to form an electrically conductive, fluorescent screen or coating 24, as shown'in Fig. 8, wherein there is a preponderance of metallic particles in the base portion or lower half of the coating, and a preponderance of fluorescent particles in the surface portion or upper half of the coating which is struck by the cathode ray.

The clear liquor l8, shown in Fig. 8, is then removed as before by draining or in any other suitable manner.

Where it occurs that the respective specific gravities of the two materials do not provide for desired rates of settling, these rates may be regulated in some degree by changing the size of the particles of one or both of the two mate-, rials, since the rate of settling is directly proportional to the square of the radius of the particles, as shown in the above formula. It will therefore be seen that by adjusting the respective sizes of the particles of the two materials, the difference between the rates of settling thereof may be made greater or less, as desired, to regulate the percentage of distribution of the fluorescent and metallic particles in the coating 24.

The suspending liquor made to cover the edge portion 25 of the heavy silver coating i3, as indicated by the broken line 25 in Fig. 10. The edge portion 21 of the screen 25 formed by settling of the fluorescent and metallic particles, therefore overlaps and'is in good electrical contact with the edge-portion 25 of the silver coating i3. I

The metallic particles 23 embedded in and forming an integral part of the translucent, fluorescent coating 24, make the same electrically conductive in the same sense and for the same ing the coating i3,

purpose as the thin coating IS in the first method illustrated in Figs. '3, 4 and 5. In the second method just described, therefore, there is no step in which a thin silver coating, such as the coating [5 in Figs. 3 and 4, is first formed on the end wall ll.

The fluorescent screen may be m'ade'electrically conductive by utilizing for the suspending liquor an alkaline solution of silver nitrate, and adding thereto a suitable reducing agent just prior to the step of covering the surface of the end wall H with the liquor. This silver solution is relatively weak as compared to that used in depositand, as shown in Fig. 9, causes a thin silver coating 28 to be deposited over each of the fluorescent particles 29 as the same settle. Simultaneously with this action, a thin silver coating, similar to coating 15 in Figs. 3 and 4,

' is deposited over the surface of the end wall ll.

In this case, also, the suspending liquor is made to cover the edge portion 25 of the relatively thick silver coating I3, so that the outer edge portion of the screen comprised of the silver-coated fluorescent particles overlaps and is in good electrical contact with the coating 13, whereby the electron current is conducted away by way of the screen, the coating l3 and connection 2|.

As a modification of the method just described, the fluorescent particles may be coated with electrically conductive material before being placed in the suspending liquor, in which case the latter would be water or some other inactive liquid. In this case, the fluorescent particles are shaken in a relatively weak silver solution, for example,

-the silver solution, placed in suspension in water l8inFigs.6and7is or some othersuitable liquid, and the method proceeded with as in that illustrated in Figs. 2 to 5.

For the fluorescent material, I propose to use zinc sulphide, zinc silicate, or so-called wfllemite, calcium tungstate, or any other suitable material. When zinc sulphide is used, I propose to use alcohol for the suspending liquor, since this material is soluble in water.

In cases where the clear liquor is removed by evaporation after settlement of the particles, ethgr or some other more volatile liquid may be use It is understood that embodiments and modiflcations of my invention, other than those shown and described, might be developed without departing from the spirit of my invention or the scope of the claims.

I claim as my invention: 1. In the art of making a fluorescent screen, the method which comprises covering a substantially horizontally disposed surface of material with a liquor holding in suspension relatively small particles of fluorescent material, allowing said particles to settle upon said surface and adhere thereto, and subsequently imparting rotary movement to said first named material at a substantially constant rate of the order of six degrees in one minute to gradually tilt said surface out of the horizontal plane.

2. In the art of cathode ray tube construction, the method of applying to structure forming part of the tube a fluorescent and electrically conductive screen, said method comprising mixing relatively small particles of fluorescent material with relatively small particles of electrically conductive material, the amount of fluorescent material in such mixture being substantially greater than the amount of electrically conductive material, placing such mixture in suspension in a liquor, applying the liquor and said particles suspended therein to said structure, and allowing said particles to settle out of suspension in said liquor into adherence with said structure and/or with each other and until the suspending liquor is substantially clear of all of said particles previously held in suspension. 7

3. In the art of cathode ray tube construction, the method of applying to structure forming part of the tube a fluorescent and electrically conductive screen, said method comprising mixing relatively small particles of fluorescent material with relatively small particles of electrically conductive material, the electrically conductive par ticles being smaller than the fluorescent particles, placing such mixture in suspension in a' liquor, applying the liquor and said particles suspended therein to said structure and allowing said particles to settle out of suspension in said liquor into adherence with said structure to form said screen.

4. In the art of cathode ray tube construction, the method of applying to structure forming part of the tube a fluorescent and electrically conductive screen, said method comprising mixing relatively small particles of fluorescent material rial, the electrically conductive particles. being smaller than the fluorescent particles, placing such mixture in suspension in a liquor, applying the liquor and said particles suspended therein to said structure and allowing said particles to settle out of suspension in said liquor into ad;-

herence with said structure to form said screen. 5. In the art of cathode ray'tube construction, the method of applying to structure forming part of the tube a fluorescent and electrically conpension the particles of fluorescent material pre-' viously coated with electrically conductive material, and allowing the coated particles to settle out of suspension from said liquor into adherence with said structure to form said screen.

7. In the art of cathode ray tube construction,

the method of applying to structure forming part of the tube a fluorescent and electrically conductive screen, said method comprising covering said structure with a silver solution holding in suspension particles of fluorescent material and containing an agent efiective to reduce said solution to metallic silver, and allowing said particles to settle out of suspension into adherence with the silver-coated surface of said structure. 8. A- fluorescent screen comprising a supporting layer, anxelectrically conductive layer carried thereupon,- and a layer of fluorescent material superposed on; the conducting layer, said fluorescent material, consisting of-discrete particles of a size progressively. decreasing in the direction away I from the supporting layer.

9. A- fluore'scent screen comprising a supporting layer, anda fluorescent layer thereupon consistqk-ing of individuai-particlesjof fluorescent material individually coatedwith a conducting layer.

1 l0. The process ofmalring 'afluorescent screen which comprisescovering. a'--substantially horidisposedbaclringtelenientwith a liquid holding in suspension pulverulent fluorescent material, permitting said material to settle out of the liquid onto the backing element to form an adherent layer and subsequently tilting said backing element away from horizontal at a sufllciently slow rate to cause the supernatant liquid to flow away without disturbing the layer of fluorescent material.

11. The invention set forth in claim wherein the tilting is accomplished at a rate substantially no greater than six degrees per minute.

12. The method of manufacturing a fluorescent screen which comprises suspending pulverulent fluorescent material and fine metallic particles in a liquid, covering a substantially horizontally disposed supporting element with the suspension, permitting the material and particles to settle out of the liquid to form an adherent layer on the supporting element and subsequently tilting said supporting element from horizontal at a rate sufficiently slow to cause the supernatant liquid to flow away without disturbing the said layer.

13. The method set forth in claim 12 wherein the sizes of the particles of the fluorescent material and the specific gravity thereof are so chosen with respect to the same characteristics of the fine '1 metallic particles that the latter settle more rapidly than the former to thereby form a layer of conductive niaterial intermediate the layer 01' fluorescent material ancl'the supporting element.

14. Thesteps in the process of manufacturing a cathode ray tube which include applying a metallic coating to a portion of the inner surface of I a container, covering a portion of the uncoated interior surface as well as a portion of the coated surface with a liquid holding'in suspension pulverulent fluorescent material and flne metallic particles, so orienting the container that the fluorescent material and the metallic particles are permitted tosettle down over the portions covered by the liquid to form an adherent layer and subsequently so tilting the container at a suificiently slow rate that the supernatant liquid flows away without disturbing the said layer.

JOHN C. BATCHELOR. 

